Proteorhodopsins are integral membrane proteins; they are isolated from uncultivated marine eubacteria and function as light-driven proton pumps. Upon absorption of light by the all-trans-retinal co-factor, proteorhodopsin goes through a photocycle with a number of intermediates. It is believed that upon excitation of the proteorhodopsin molecule by light stimulation, a proteorhodopsin/retinal complex is excited to an unstable intermediate energy state. Proteorhodopsin progresses through a series of unstable energy states that can vary in terms of energy plateaus or intermediates, e.g., an “M-like state” or “M-state”, a “K-like state” or “K-state”, an “N-like state” or “N-state”, or an “O-like state” or “O-state”. Subsequently, the complex reverts to a more stable basal state concomitant with transportation of a proton.
Proteorhodopsins are distantly related to bacteriorhodopsin from Halobacterium salinarium (22-24% sequence similarity). Hampp (Appl. Microbiol. Biotechnol. 53:633-9, 2000a) reviews the structure and function of bacteriorhodopsin, and its technical applications. Hampp (Chem. Rev. 100:1755-76, 2000b) reviews the technical application of bacteriorhodopsin.
Proteorhodopsin and bacteriorhodopsin have some shared characteristics, but also have clearly different properties. Proteorhodopsins are more advantageous to use in some technical applications than bacteriorhodopsins because of the ease of expressing and producing proteorhodopsins. However, the conditions where the proteorhodopsins can be used in different applications are limited because wild-type proteorhodopsins exist in two distinct spectral forms depending on the extra-cellular pH. A basic form, which is a spectral form at a higher pH, is able to achieve an M-state of excitation and transport a proton upon exposure to an optical stimulation. An acidic form, which is a spectral form at a lower pH, is unable to exhibit the M-state of excitation and does not transport a proton upon exposure to an optical stimulation.
The properties of the two distinct pH-dependent spectral forms of the Bac31A8 proteorhodopsin have been characterized to some extent (Dioumaev, et al., Biochem. 41:5348-58, 2002; Krebs, et al., BMC Physiol. 2:1-8, 2002; Fredrich, et al., J. Mol. Biol. 321, 821-838, 2002). The D97 residue in the Bac31A8 proteorhodopsin was previously identified (Dioumaev, et al., 2002) as being part of the titratable group(s) involved in the pH dependent change in spectral and photochemical properties. However, the Bac31A8 D97N mutant protein appears only to exist as a single spectral form, the acidic form. An analysis of the photocycle intermediates of the Bac31A8 proteorhodopsin at different pH values showed that only the high pH (“basic”) form exhibits the photocycle wherein protons are pumped across the membrane. Hence, this Bac31A8 D97N mutant is not very useful for most applications because the protein is unable to pump protons and form an M-state.
Béjà, et al. (Science 289:1902-6, 2000) disclose the cloning of a proteorhodopsin gene from an uncultivated member of the marine γ-proteobacteria (i.e., the “SAR86” group). The proteorhodopsin was functionally expressed in E.coli and bound all-trans-retinal to form an active light-driven proton pump.
Béjà, et al. (Nature 411:786-9, 2001) disclose the cloning of over twenty variant proteorhodopsin genes from various sources. The proteorhodopsin variants appear to belong to an extensive family of globally distributed proteorhodopsin variants that maximally absorb light at different wavelengths.
WO 01/83701 discloses specific proteorhodopsin gene and protein sequences retrieved from naturally occurring bacteria; the reference also discloses the use of these proteorhodopsin variants in a light-driven energy generation system.
Dioumaev, et al. (Biochem. 41:5348-58); Krebs, et al. (BMC Physiol. 2:1-8, 2002); and Friedrich, et al (J. Mol. Biol. 321, 821-38, 2002) disclose the properties of two distinct pH-dependent spectral forms of the Bac31A8 proteorhodopsin. Dioumaev, et al. also disclose that essentially only the acidic form is present in the Bac31A8 D97N mutant since the N residue is non-protonatable. Further, Dioumaev, et al. disclose that the D97E mutant causes minor changes in the absorbance maximum of the acidic and basic forms. An E108Q mutant causes the decay of the M-like state intermediate to be a hundred fold slower. Both the D97E and E108Q mutants have pH titration similar to that of the wild-type protein.
Varo, et al. (Bioophysical J., 84:1202-1207 (2003)) describe the results of a thorough analysis of the photocyle of the wildtype Bac31A8 proteorhodopsin; the spectral properties and lifetimes of different intermediates in the photocycles are characterized.
WO 02/10207 discloses proton-translocating retinal protein, such as a Halobacterium salinarim bacteriorhodopsin, in which one or more positions of the amino acids that participate in proton-translocation, from the group of amino acid residues D38, R82, D85, D96, D102, D104, E194 and E204 are modified; such proton-translocating retinal proteins have a slower photocycle in comparison to with the wild-type proteins.